Various coffee decaffeination methods are well-known in the art. The most common techniques involve first swelling the coffee beans with water and then extracting the caffeine with an organic solvent or a caffeine-deficient solution of green coffee solubles which solution is then itself contacted with a solvent to remove the caffeine therefrom. In either case, at least some of the solvent typically contacts the beans, leaving minute traces therein. The most useful solvents are halogenated hydrocarbons, but it is becoming increasingly desirable to avoid such solvents so as to leave the coffee free of any trace solvent.
One of the more promising, although costly, alternative techniques is the use of a supercritical fluid, preferably supercritical carbon dioxide, to extract the caffeine from green coffee beans. Such a technique is disclosed in U.S. Pat. No. 4,260,639 to Zosel wherein green coffee is contacted with water-moist supercritical carbon dioxide in order to extract the caffeine. The caffeine may be absorbed from the caffeine-laden supercritical carbon dioxide by bubbling the carbon dioxide through a water reservoir, said reservoir being replaced by fresh water every 4 hours, as disclosed in U.S. Pat. No. 3,806,619 to Zosel. However, such a recovery system is highly inefficient because the water reservoir fails to provide a continuous driving force for caffeine recovery and the periodic replacement of the reservoir results in an undesirable discontinuity in the process.
In still another technique, disclosed in U.S. Pat. No. 4,247,570 to Zosel, the green coffee is mixed with a caffeine adsorbent prior to contact of the coffee and the supercritical fluid. Then, as the caffeine is extracted by the supercritical fluid, it is adsorbed by the caffeine adsorbent, eliminating the need for a separate caffeine removal step. The prior art methods are batch techniques which tend to be less efficient than would be more nearly continuous methods. In addition, loss of non-caffeine solids in solid absorbents and in purging the system adversely effects the green coffee roasted flavor.
An advantage of the present invention is a more nearly continuous method of extracting caffeine from green coffee beans with a supercritical fluid removal of the caffeine and recovery and recycle of non-caffeine solids to the green coffee.
Another advantage is to produce a decaffeinated coffee of improved quality by limiting the loss of non-caffeine solids during decaffeination and by decreasing substantially the residence time of green beans in the process.